The Amino Acid We Stopped Eating
For most of human history, people consumed the whole animal — not just the muscle meat. Skin, tendons, bones, cartilage, and organ meats were eaten regularly. These tissues are rich in collagen, and collagen is the most glycine-dense protein in the body: roughly one third of collagen's amino acid content is glycine.
Modern diets have largely eliminated these cuts in favor of boneless, skinless muscle meat. The result: most people eating a modern diet consume 2–3g of glycine per day — far below the 10–12g estimated as optimal for supporting all of glycine's physiological functions. This gap is now recognized as a meaningful nutritional shortfall.
What Glycine Does
Collagen synthesis — Glycine is the rate-limiting amino acid in collagen production. Collagen is the structural protein in skin, joints, tendons, ligaments, bones, and the gut lining. Without sufficient glycine, collagen synthesis slows — affecting joint health, skin elasticity, wound healing, and gut barrier integrity.
Sleep quality — This is glycine's most well-documented acute effect. A landmark Japanese study found that 3g of glycine before bed significantly improved subjective sleep quality, reduced sleep onset time, and decreased daytime sleepiness the following day. Mechanistically, glycine lowers core body temperature by dilating peripheral blood vessels — and dropping core body temperature is one of the primary triggers for sleep onset.
Detoxification — Glycine is essential for glutathione synthesis (your body's master antioxidant) and for conjugating bile acids that eliminate toxins via the liver. Adequate glycine supports both processes.
Muscle protein synthesis — While not a branched-chain amino acid, glycine is a significant component of muscle tissue and is required for the full utilization of other amino acids in muscle repair. Studies suggest glycine supplementation modestly improves lean mass retention, particularly during calorie restriction.
Gut health — Glycine strengthens tight junctions in the intestinal wall, helping to maintain the gut barrier and reduce intestinal permeability ("leaky gut"). It also modulates gut inflammation via macrophage regulation.
Blood sugar regulation — Glycine stimulates glucagon-like peptide-1 (GLP-1) release, slowing gastric emptying and improving insulin sensitivity. Epidemiological studies have linked higher dietary glycine to lower risk of type 2 diabetes.
Dosage
For sleep: 3g, taken 30–60 minutes before bed. This is the most robustly studied dose for sleep improvement.
For collagen support: 5–10g daily, ideally paired with vitamin C (which is required for collagen cross-linking). Can be split across meals.
General supplementation: 3–5g/day is well-tolerated with no known adverse effects at these doses.
Food sources: Bone broth (1–2g per cup), collagen powder (typically 3–4g glycine per 10g serving), gelatin, skin-on poultry, and organ meats. Collagen powder is the most practical food-based source for those not regularly consuming connective tissue-rich foods.
Interaction with Creatine
Glycine is one of the three amino acids your body uses to synthesize creatine endogenously. When you supplement creatine, you reduce the demand on your body's glycine supply — potentially freeing up glycine for its other functions. Some researchers suggest this is one underappreciated benefit of creatine supplementation.
The Bottom Line
Glycine is conditionally essential — your body makes it, but not in amounts sufficient for optimal function when diet is poor in connective tissue foods. 3g before bed for sleep, 5–10g across the day for collagen and general health, is a low-cost, high-return supplementation strategy with an excellent safety profile and decades of supportive research.